Mechanical filter

A mechanical filter is a type of designs allowing to filter out gases and liquids via their mechanical behavior instead of specialized buildings. Those patterns tend to work passively, rely on no power at all, but their efficiency might vary depending on situation.

Utilizing layering
Both gases and liquids tend to naturally sort themselves out into layers based on their densities and pressures if left undisturbed, which allows for building of low maintenance catchment and overflow structures.

The most basic example of that is Carbon Dioxide pits used to store food, which fill themselves with natural output of duplicants or any Carbon Dioxide falling from the upper levels. It can be extended to "industrial" pit, where generators and machinery are placed below the base habitation level, which provides passive heat insulation via natural properties of denser gasses.

It is mirrored by hydrogen catchers, built inside SPOMs and incorporated into upper levels of bases, which allow to use only one occasionally run pump to collect any and all incoming hydrogen.

Liquid-related design are trickier, but still possible, since clear water always sort itself over polluted and liquids tend to stick inside narrow, one-tile passages. But mostly it is used as a base for Liquid Airlocks and gas separation.

Tile-based mechanical filters demand non-localized approach and stable conditions or they can be overwhelmed by minutia change, for example fluctuations in oxygen pressure might lead to chlorine and CO2 escaping from upper-layer pits into lower layers, which in turn makes it even less breathable there.

Inside pipes
Mechanical filters inside pipes tend to utilize their "packet" nature by pre-seeding and constantly looping tiny packets of desirable material, which blocks passage for any other materials.

Pipe-based mechanical filters might limit overall pipe-throughput and are vulnerable to clogging, if designed carelessly.

External resources

 * https://forums.kleientertainment.com/forums/topic/83427-how-to-filter-multiple-fluids-at-once/
 * https://www.youtube.com/watch?v=likVZxoyfno